Significant improvements in programmable microcontrollers over the past five years as well as the existence of dimming systems which use complex algorithms have caused most major electronic ballast companies to develop microcontroller driven electronic ballasts. These electronic ballasts typically use microcontrollers to adjust the characteristics of the inverter voltage signal to accommodate a wide variety of lamps and/or to provide dimming functionality. Specifically, by changing the frequency or duty cycle of the inverter voltage signal, these electronic ballasts are able to start, run and dim a wide variety of gas discharge lamps.
Some electronic ballasts such as the one disclosed by U.S. Pat. No. 5,039,921 to Kakitani uses a central processing unit (CPU) to control the frequency of the inverter voltage signal to change lamp voltage. The Kakitani patent describes a ballast which can be adapted to light and drive various types of gas discharge lamps according to each lamp's individual rating. The control circuit employs the CPU to detect the rating of the discharge lamp based on the lamp's starting voltage and to retrieve stored lamp loading data from memory relating to the type of discharge lamp detected. The oscillating frequency of the inverter circuit voltage signal is then adjusted so that the ballast produces a power voltage signal suited to the particular discharge lamp.
Other electronic ballasts such as the one disclosed by U.S. Pat. No. 5,569,984 to Holtstag use a microprocessor to control the switching frequency and the pulse width of the inverter voltage signal provided to a particular lamp to avoid strong acoustic resonances or arc instabilities. The microprocessor evaluates deviation of electrical lamp parameters to detect arc instabilities and adjusts the frequency and pulse width in response. Accordingly, the ballast can operate HID lamps of different types, wattages and manufacturers over a broad frequency range despite the occurrence of acoustic resonance/arc instabilities among these lamps.
In order to achieve acceptable levels of accuracy in running and dimming a wide variety of gas-discharge lamps, it is necessary to be able to produce a wide variety of inverter voltage signals which requires a high resolution of control signals. Low-speed microcontrollers cannot provide the necessary degree of control to run a lamp within a ballast having conventional inverter signal frequencies. In order to achieve the desired operation of a typical ballast, expensive high-speed microcontrollers must be used which severely limits mass production and consumption of microcontroller-based electronic ballasts due to the cost of such high-speed microcontrollers.
Further, since microcontrollers provide discreet output, when digital output levels are provided to a lamp, sudden incremental changes in the lumen output are produced. These discrete "steps" in light intensity are visible to users and are unacceptable in commercial and residential environments. Even when the microcontroller is programmed to dim a lamp in relatively small increments, dimming a lamp using a digital signal still results in visible steps.
Finally, in order to provide sufficient power supply to the microcontroller, either a drop-down resistor or a dedicated off-line power supply circuitry is used. The problem with using a simple voltage-drop resistor is that the heat and high frequency noise which are generated are very difficult to suppress. On the other hand, a separate off-line power supply adds substantial expense to the product.
Thus, there is a need for a universal lighting ballast control circuit which can produce a wide range of different control signals to start, run and dim a wide variety of gas-discharge lamp types using an inexpensive low-speed microcontroller, which can modulate illumination levels on a continuously variable basis and which provides power to the microcontroller without conventionally known power supply problems and associated expense.